I've been publishing software on the internet for a long time. Most of it wasn't very exciting, nor especially useful. Why did I choose the GPL license from about 1999 onwards? The reasons were not very complicated.

Prior to 1999 I just uploaded code to a website without any licenses. It was mainly small demos for technologies which are now thoroughly obsolete. Then at some point in 1999, or maybe 2000, someone emailed saying something like: I see you are publishing source code. Unless you add a license with a warranty disclaimer someone might try to sue you. Also without a license this isn't public, it's all rights reserved. I wasn't very interested in legal stuff and so I did a bit of reading and found that GPL best matched what I was doing. My thinking was that if I'm putting software out there with the intention of it being public then I'd rather that it remained public and not have any proprietary forks. That way if there are improvements I can incorporate them back into the original. The sharealike nature of GPL fitted that goal.

There are other reasons to use copyleft licenses, but this is still mostly my reasoning about it now. Other kinds of licenses seem to have more down sides to them if the goal is a global public software commons.